The restriction (endonucleolytic attack) and modification (methylation) of DNA are related, general mechanisms that are widespread in bacteria. We are investigating several types of restriction and modification systems associated with bacteria, bacterial viruses and plasmids such as R-factors. The investigations involve genetic and biochemical analyses and our aim is to elucidate the nature of the molecular events in these processes. We are also exploiting the enzymes of these systems as a probe for investigating protein-DNA interactions and as tools for studying small DNA genomes (gamma, SV40, R-factor). Several features of one of the endonucleases (Eco RI) are being applied to the construction of bacterial plasmids.